Scalable Secure Scuttlebutt

• @dominictarr

• scuttlebutt.nz

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Scuttlebutt

"anti-entropy" gossip protocol, part of amazon dynamo

• "flow control for anti-entropy protocols"
• eventually consistent
• used by within a trusted system

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Secure Scuttlebutt

• peer id is public key
• data model signed hash chains
• peers can relay other's messages
• cannot insert, modify, or reorder messages
• large objects in optional "attachments"

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• subscribe ("follow") model solves sybil attacks
• provides social discovery
• messages arrive in natural order - easy to build database on top of

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Scalable Secure Scuttlebutt

• optimized for bandwidth and latency
• overhead proportional to feeds updated

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part 1: network topology

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Star Network

naive design: connect to everyone

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return require('./demos').centralized()

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Random Network

• connect to N > 2 peers ~= fully connected
• tradeoff between bandwidth & latency

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return require('./demos').random()

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Rules for gossip flooding

send new messages to everyone that didn't send it to you

(don't resend messages you already know!)

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message passing diagram

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bandwidth complexity

O(messages*(spanning+2*redundant))

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how many redundant connections

• 1: 0%
• 2: 66%
• 3: 80%
• 4: 86%
• 5: 89%
• 10: 95%

at 5 connections per peer, we use 9 times as much bandwidth

Spanning Tree Network

remove duplicate connections

• when you receive an old message, disable that connection
• bandwidth complexity now approaches O(messages)

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return require('./demos').spanning()

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trees are fragile

return require('./demos').fragile()

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spanning tree

• network partitions are really easy
• we just added many points of failure!

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Epidemic Broadcast Trees

• (from paper with the same name)

• best of flooding and spanning

• we run a separate tree per feed

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Eager and Lazy (aka push and pull)

• connections have two modes: eager and lazy
• switch redundant connections into lazy
• eager mode pushes message immediately
• lazy mode sends short note that message exists

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• if you receive a known message, ask them to be lazy
• (if in lazy mode, just send a short note)
• if you receive a note for a new message, switch connection back to eager

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state per feed

• feed id (string)
• local sequence (integer)
• remote sequence (integer)
• local request (integer)
• remote request (integer)
• local mode (boolean)
• remote mode (boolean)

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part 2: handshakes

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basic vector clock

(as described in flow gossip paper)

Alice: {A: 1, B: 0, C: 3}

Bob:   {A: 0, B: 1, C: 3}

alice sends A1, Bob sends B1 handshake size is O(feeds)

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skipping vector clock

Alice stores Bob's last vector clock

Alice (local): {A: 2, B: 2, C: 3}

Bob (stored):  {A: 0, B: 1, C: 3}

drops elements which havn't changed (Carol hasn't updated)

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Note: Alice stores Bob's A:0 even though she sent A1. unless Bob acknowledges A1, Alice doesn't know for sure he has it.

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so Alice sends:

Alice: {A: 2, B: 2}

Bob: {A: 1, B: 3}

Alice sends A2, Bob sends B3

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if Bob then gossips with Carol

Alice: {}

Bob:   {C: 4}

Alice sends an empty clock, because she thinks nothing has changed

but Bob has an update from Carol

Alice: {C: 3}

and Bob sends C4

so, it costs one more message pass, but usually saves lots of requests.

note, we also save tracking this in memory

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distribution of updates

• power law is typical
• a few active users
• some moderate users
• many occasional / infrequent users

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• basic scuttlebutt

  O(messages + feeds)

• scalable scuttlebutt

  O(messages + updated_feeds)

  (updated_users is probably much smaller)

  reconnections are very cheap!

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complexity overview

bandwidth: O(messages + updated_feeds)

latency: replication in 1 or 2 round trips

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Part 3: Comparison to IPFS

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• replicate data without global consistency
• secure scuttlebutt optimizes for chains
• this is worst case for ipfs

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• ssb streams updates in order, per feed
• ipfs has pointer to latest message then works backwards, one message at a time

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• ipfs better for arbitary partial datasets
• ssb better for social applications
• ssb: database vs ipfs: file system

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Thank you!

• learn more: https://scuttlebutt.nz
• frontend: https://github.com/ssbc/patchwork
• backend: https://github.com/ssbc/scuttlebot

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